I have a question for the fic writers among you. What are your opinions on OCs? What makes a good original character and what stops them being Mary Sues?

We've all seen the fics where a mysterious stranger comes into town and sweeps our poor downtrodden hero off his/her feet whilst single-handedly saving the world and generally being an all-round good egg. Not all fics featuring OCs are so predictable though, I know a lot of you have written some fantastic stories featuring original characters so how do you avoid the Mary Sue pit falls? Is it a conscious thought process to avoid them or is it something else?

And for the fic readers among you, what do you think of OCs? Does the mention of an original character put you off reading a fic?

I'm just full of questions today aren't I?!